1. Field of the Invention
The present invention relates to an improvement of a thermosiphon for use in a liquefying plant wherein a fluid to be cooled is subjected to precooling for liquefaction, and more particularly to a space-saving type and plate fin heat exchanger built-in type multi-stage thermosiphon which permits reduction in the number of pipes for a fluid to be cooled and for a refrigerant and thereby permits reduction in size of the apparatus.
2. Description of the Related Art
In a natural gas liquefying plant for liquefying a fluid to be cooled, e.g. natural gas, there is disposed a multi-stage thermosiphon for precooling the natural gas to effect liquefaction. As such multi-stage thermosiphons there are known one using a plate fin type heat exchanger 50 which will be described later and one using a shell and tube type heat exchanger in which a plurality of heat exchanging tubes are disposed in parallel.
Description is now directed to the construction of the typical plate fin type heat exchanger 50, which is illustrated as a perspective view of its principal portion in FIG. 5. As shown in the same figure, plural plate fins 51 formed in a corrugated shape and flat plates 52 are laminated in an alternate manner, and between the flat plates 52 there are formed to-be-cooled fluid flow paths 53 and refrigerant circulation paths 54 in an alternately manner. A fluid to be cooled is allowed to flow in a direction same as or opposite to a circulating direction of a refrigerant. In the plate fin type heat exchanger 50, as shown in FIG. 6 which is a partially cut-away perspective view of the heat exchanger, a plurality of corrugated plate fins 51 are disposed between the to-be-cooled fluid flow paths 53 in a direction orthogonal to the flow paths. Further, partition plates 55 are disposed at predetermined intervals to define a plurality of independent groups of refrigerant circulation paths 54, and the refrigerant is allowed to circulate in a direction orthogonal to the flowing direction of the fluid to be cooled.
Examples of multi-stage thermosiphons will now be described with reference to FIGS. 7 and 8, in which the same reference numerals as above are used with respect to the construction of a plate fin type heat exchanger used therein. FIG. 7 is a schematic diagram of a plate fin heat exchanger built-in type multi-stage thermosiphon according to a first prior art example, in which refrigerant tanks are externally provided. More specifically, the reference numeral 50 in the same figure denotes a plate fin type heat exchanger. In the heat exchanger 50, a plurality of independent refrigerant circulation paths 54 formed between to-be-cooled fluid paths 53 are partitioned by partition plates disposed in a direction orthogonal to the fluid paths 53, though not shown. Refrigerant supply pipes 62 and refrigerant return pipes 63 are in communication with the refrigerant circulation paths 54 from three refrigerant tanks 61 each provided with a refrigerant supply port 61a for the supply of, for example, liquid propane gas (hereinafter referred to as the "refrigerant") and also provided with a refrigerant return port 61b for return of the refrigerant which is in a gaseous or gas-liquid state. Under this construction, while natural gas (hereinafter referred to as the "fluid to be cooled" or "to-be-cooled fluid") flows through the to-be-cooled fluid flow paths 53 in the plate fin type heat exchanger 50, it is precooled by the refrigerant which is circulated in the refrigerant circulation paths from the refrigerant tanks 61 through the refrigerant supply pipes 62, then flows out and is fed to the next cooling process (not shown). The refrigerant in each refrigerant tank 61 absorbs heat from the to-be-cooled fluid, vaporizes partially, and the vaporized refrigerant is sucked from a refrigerant suction port 61d, while the remaining refrigerant again stays as liquid within the refrigerant tank 61. As to replenishment of the refrigerant into the tank 61, it is performed through a refrigerant replenishing port 61c.
FIG. 8 shows the construction of a shell and tube heat exchanger type thermosiphon according to a second example of the prior art, in which a refrigerant tank 71 is disposed horizontally. Though not shown, a plurality of refrigerant tanks 71 are connected in series through a to-be-cooled fluid pipe 72 to constitute a multi-stage thermosiphon. Within each refrigerant tank 71 is disposed a tube bundle 70 having a to-be-cooled fluid flow path comprising a plurality of bent tubes which are in communication with both an inlet port 71a formed on one end side for the admission of natural gas as the fluid to be cooled and an outlet port 71b formed on an opposite end side for the discharge of the natural gas after precooling. The tube bundle 70 is immersed in a refrigerant which has been introduced from a refrigerant supply port 70c. Therefore, the fluid to be cooled flowing into the tank through the inlet port 71a is cooled while passing through the to-be-cooled fluid flow path, then passes through the fluid pipe 72 connected to the outlet port 71b and flows into the thermosiphon located on the downstream side, in which the fluid is again precooled. The thus-precooled fluid which has flowed out from the outlet port 71b of the thermosiphon located as the last-stage siphon on the downstream side is fed to the next cooling process.
In the plate fin heat exchanger type multi-phase thermosiphon according to the first prior art example described above, the refrigerant tanks and the plate fin type heat exchanger are arranged separately. Consequently, it is necessary to provide refrigerant supply pipes and return pipes for the circulation of the refrigerant between the refrigerant tanks and the heat exchanger, thus inevitably resulting in increase in the size of such a multi-stage thermosiphon. Further, the pressure of the refrigerant becomes low due to pressure loss in such pipes, so that the boiling point of the refrigerant rises and the cooling performance of the multi-stage thermosiphon is deteriorated. For ensuring a predetermined cooling performance, therefore, it is required to increase the size of the multi-stage thermosiphon. This problem remains to be solved.
In the shell and tube heat exchanger type thermosiphon according to the second example of the prior art described above, since the flow path of the fluid to be cooled is formed within each refrigerant tank, it is not necessary to use such refrigerant supply pipes and return pipes as in the first prior art example. However, the shell and tube type heat exchanger is inferior in the heat exchange performance to the plate fin type heat exchanger. In addition, it is necessary to use a pipe for the flow of fluid to be cooled from an upstream-side refrigerant tank to a downstream-side tank. Thus, like the first conventional example, an increase in the size of the multi-stage thermosiphon is unavoidable. Further, since the fluid to be cooled undergoes a phase change, that is, it is condensed into a two-phase flow, it is necessary that the two-phase flow of the fluid flowing from an upstream-side refrigerant tank to a downstream-side tank through the to-be-cooled fluid pipe be flowed in a uniformly distributed state through the downstream-side thermosiphon. However, such uniform distribution of the two-phase flow is extremely difficult and so there is a fear of deterioration in the cooling performance of the multi-stage thermosiphon. This problem also remains to be solved.